1. Field of the Invention
This invention relates to magnetic disk drives. In particular, embodiments of the present invention relate to disk drives, head stack assemblies and actuator arm assemblies that include a bobbin to stiffen the coil portion of an actuator.
2. Description of the Prior Art and Related Information
A typical hard disk drive includes a head disk assembly (“HDA”) and a printed circuit board assembly (“PCBA”). The HDA includes at least one magnetic disk (“disk”), a spindle motor for rotating the disk, and a head stack assembly (“HSA”) that includes a slider with at least one transducer or read/write element for reading and writing data. The HSA is controllably positioned by a servo system in order to read or write information from or to particular tracks on the disk. The typical HSA has three primary portions: (1) an actuator assembly that moves in response to the servo control system; (2) a head gimbal assembly (“HGA”) that extends from the actuator assembly and biases the slider toward the disk; and (3) a flex cable assembly that provides an electrical interconnect with minimal constraint on movement.
A typical HGA includes a load beam, a gimbal attached to an end of the load beam, and a slider attached to the gimbal. The load beam has a spring function that provides a “gram load” biasing force and a hinge function that permits the slider to follow the surface contour of the spinning disk. The load beam has an actuator end that connects to the actuator arm and a gimbal end that connects to the gimbal that supports the slider and transmits the gram load biasing force to the slider to “load” the slider against the disk. A rapidly spinning disk develops a laminar airflow above its surface that lifts the slider away from the disk in opposition to the gram load biasing force. The slider is said to be “flying” over the disk when in this state.
FIG. 1 shows an example of a conventional actuator assembly 10. As shown therein, the conventional actuator assembly 10 includes a body portion 12 from which are cantilevered one or more actuator arms 14. Also cantilevered from the actuator body portion 12 is a coil portion that includes first and second actuator fork members 16 and 18. The actuator fork members 16 and 18 support the wound coil 22 that forms a portion of the disk drive's actuator coil portion. The wound coil 22 is also at least partially encased by a plastic overmold 20, which further supports and adds rigidity to the coil 22 and to the actuator assembly 10. As shown, the actuator assembly 10 also includes a bobbin 24, which is secured to the coil 22 by an adhesive at 27 and which further increases the rigidity of the coil 22 and that of the actuator assembly 10.
The overmold 20 is formed using a plastic injection molding process at high temperatures. As it cools, the overmold 20 may form voids within its thickness. Such voids adversely affect the resulting rigidity of the overmold and that of the overall actuator assembly. While such decreased rigidity may nevertheless fall within acceptable operational parameters for drives destined for the consumer market, such decreased rigidity may adversely affect the operation of the higher performing drives aimed at the enterprise market. Indeed, the higher data densities and higher platter rotational speeds of such drives require a very rigid (stiff) actuator assembly, in which the bending, torsional, sway and system modes are shifted to higher frequencies, as compared to lower performing drives.
As the actuator assembly 10 is not and cannot be made to be perfectly stiff, these resonant modes occur as the actuator assembly 10 vibrates in response to a given excitation frequency or frequency range. Stiffening the actuator assembly 10, all other aspects thereof remaining the same, tends to beneficially increase the frequencies at which such bending vibrations occur and tends to beneficially reduce the amplitude of such vibrations. The stiffer the actuator assembly 10 can be made, the higher the frequencies will be at which it will bend responsive to a given excitation frequency or frequency range.
Such resonant modes interfere with the drive's reading and writing activities, and typically degrade the drive's seek time performance. To address such resonant modes, a notch filter or filters tuned to the resonant mode frequencies may be used in the drive's servo to attenuate signals at these frequencies, to the detriment of available servo bandwidth. Moreover, it is easier to attenuate higher frequencies without unacceptable loss of signal amplitude, as it is to attenuate unwanted resonant mode frequencies at comparatively lower frequencies. From the foregoing, it may be appreciated that there is a clear need for shifting the resonant mode frequencies (such as the pivot butterfly frequency) higher and/or to eliminate one or more resonant modes of actuator assemblies of hard disk drives. Doing so would decrease drive seek times and decrease the degradation of servo bandwidth caused by such resonant modes, among other benefits. As the VCM is driven with ever-higher currents to reduce seek times, thermal dissipation in the VCM becomes a non-negligible issue. Indeed, heat is generated as the coil of the VCM is subjected to high coil driving currents and this heat must be dissipated. A need has developed, therefore, to find means for efficiently dissipating such heat to insure that the VCM is not damaged and may continue to be driven with such high driving currents.